Encapsulated graphite heater and process

ABSTRACT

A graphite heater and method of forming a graphite heater comprising a graphite body configured in a spiral or serpentine geometrical shape defining at least one zone of an electrical heating circuit having opposed ends so as to provide an electrical path for each heating zone through the graphite body from the opposed ends with said graphite body having at least one heating surface, a coating of pyrolytic boron nitride or aluminum nitride encapsulating said graphite body and a surface layer of pyrolytic boron nitride or aluminum nitride disposed on said graphite body such that said heating surface is continuous and solid.

FIELD OF INVENTION

[0001] The present invention relates to a heating device primarily for use in semiconductor wafer processing equipment.

BACKGROUND OF THE INVENTION

[0002] A semiconductor wafer is processed at relatively high temperature of up to about 1200° C. in the fabrication of a semiconductor device or semiconductor material. The temperature of the semiconductor wafer must be held substantially constant and uniform with thermal uniformity requirements on the order of 1° C. and 10° C. A typical semiconductor wafer heater is manufactured from graphite and is currently a platen e.g. 4 to 12 inches in diameter and 0.10 to 0.50 inches thick, or a cylinder e.g. 2″ to 20″ in inside diameter, 0.10″ to 0.50″ wall, and 2″ to 40″ long, the graphite of which is machined into a spiral or serpentine configuration defining a given area to be heated and with the machined graphite body having opposed ends connected to a source of external power. Furthermore, a multiple zone heating system is often accomplished with each zone made independently. Although graphite is a refractory material which is economical and temperature resistant, graphite is corroded by some of the wafer processing chemical environments, and it is prone to particle and dust generation. Due to the discontinuous surface of a conventionally machined graphite heater, the power density varies dramatically across the area to be heated. Moreover, a graphite body particularly after machining into a serpentine geometry is fragile and its mechanical integrity is poor and accordingly even if the cross sectional thickness is relatively large, e.g., above about 0.1 inches or greater in thickness, which is typical for semiconductor graphite heater applications, the graphite heater is still extremely weak and must be handled with care. Furthermore, over time a graphite heater changes dimension due to annealing which induces bowing or misalignment and can result in an electrical short circuit. It is also conventional in semiconductor wafer processing to deposit a film on the semiconductor which may be electrically conductive. Such films may also deposit as fugitive coatings on the graphite heater which can also contribute to an electrical short circuit or can also contribute to a change in electrical properties.

SUMMARY OF THE INVENTION

[0003] It has been discovered in accordance with the present invention that a heater can be formed from a solid graphite body overcoming the inherent fragility and weakness of a solid graphite body and still retain its refractory properties by encapsulating the solid graphite body in a thin film shell composed of pyrolytic boron nitride (pBN) or aluminum nitride and by forming a continuous surface of pyrolytic boron nitride (pBN) or aluminum nitride contiguous to the graphite body for defining a predetermined heating surface. Moreover, in this configuration, the mechanical strength of the heater is dramatically improved relative to the strength of a conventional graphite heater. In the process of the present invention the heater is fabricated to provide a continuous solid surface of pyrolytic boron nitride or aluminum nitride on one side of a serpentine machined graphite body, the continuous face eliminates the discontinuous limitation of the prior art providing improved thermal uniformity. In addition, the graphite heater of the present invention forms a single structurally integral unit which is more stable than a typical graphite heater without any exposed surfaces thereby preventing short circuits and electrical changes from occurring, and insuring a clean, dust and particle free surface. Furthermore, pyrolytic boron nitride (pBN) and aluminum nitride encapsulations can provide enhanced corrosion resistance against the wafer processing chemical environment.

[0004] The graphite heater of the present invention comprises a graphite body configured in a spiral or serpentine geometrical shape defining at least one zone of an electrical heating circuit, having opposed ends so as to provide an electrical path for each heating zone through the graphite body from the opposed ends, with said graphite body having a coating of pyrolytic boron nitride or aluminum nitride encapsulating said graphite body and a layer of pyrolytic boron nitride or aluminum nitride disposed on the graphite body to form a solid continuous heating surface.

[0005] The method of the present invention for forming a graphite heater from a solid body composed of graphite having at least one continuous surface comprises the steps of: coating said graphite body in a CVD reactor furnace to form a layer of pyrolytic boron nitride or aluminum nitride on all surfaces of the graphite body; machining said coated graphite body from the side opposite said one continuous surface to form a spiral or serpentine pattern extending through the graphite body up to said one continuous surface such that a solid continuous layer of pyrolytic boron nitride of predetermined thickness is left intact covering the machined graphite body, with each electrical circuit pattern formed in said graphite body, having opposed ends adapted for connection to an external power supply and encapsulating said machined graphite body in a material composition selected from the group consisting of pyrolytic boron nitride or aluminum nitride to form a thin film shell of said material surrounding said heater.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006] Other advantages of the present invention will become apparent from the following detailed description of the invention when read in conjunction with the accompanying drawings of which:

[0007] FIGS. 1(a), (b), (c), (d) and (e) illustrate the sequence of steps for fabricating a flat graphite heater in accordance with the present invention;

[0008]FIG. 2 is a plan view of a flat heater formed following the sequence of steps of FIGS. 1(a) through 1(e) viewed from the underside of FIG. 1 (e); and

[0009] FIGS. 3(a-c) illustrate a typical cylindrical shaped heater formed in accordance with the present invention in which FIG. 3(a) represents a top view, FIG. 3(b) shows the cylindrical heater in cross section taken along the lines A-A of FIG. 3(a) and FIG. 3(c) representing an exploded view of the encircled area shown in FIG. 3(b).

DETAILED DESCRIPTION OF THE INVENTION

[0010] The sequence of steps for fabricating a graphite heater in accordance with the present invention is illustrated in FIGS. 1(a) through 1(e) respectively. A solid body composed of any conventional graphite material is machined into a base 10 having any desired outside geometry such as rectangular or disk shaped as shown in FIG. 1(a) and having at least one continuous surface 12. The graphite body could also be a cylinder as shown in FIG. 3 or any other general heater shape. The body 10 can be of any desired thickness (d) suitable for the intended application. For use in semiconductor wafer processing the graphite base 10 should typically be of a thickness above at least about 0.10 inches.

[0011] The graphite base 10 is coated with a layer of pyrolytic boron nitride 14 as shown in FIGS. 1(b) of any desired predetermined thickness of e.g. 0.02 inches. Alternatively the graphite base 10 can be coated with aluminum nitride. In either case the coating is preferably formed by chemical vapor deposition (CVD) in a conventional manner. In this regard, pyrolytic boron nitride (pBN) is the preferred coating and is typically formed by a chemical vapor deposition process as described in U.S. Pat. No. 3,182,006, the disclosure of which is herein incorporated by reference. The process involves introducing vapors of ammonia and a gaseous boron halide such as boron trichloride (BCl₃) in a suitable ratio to form a boron nitride deposit on the surface of an appropriate substrate which in the present invention would be the graphite base 10. Pyrolytic boron nitride is a dielectric material which is very anisotropic having a thermal conductivity in the plane (planar surface 12) of typically 30 times or more above the thermal conductivity through the thickness.

[0012] The coated graphite base 10 is then machined from the side opposite the continuous surface 12 into a desired electrical pattern for resistive heating preferably that of a spiral or serpentine configuration 13 as shown in FIG. 2 having opposite open ends 15 and 16 adapted for connection to an external power supply. The machining operation is conducted through the cross sectional body of the graphite base 10 up to the pBN coating 14 on the continuous surface 12 without removing the pBN coating 14 covering the continuous surface 12 i.e., the pBN coating is left intact. The pBN coating 14 remains to form a flat solid continuous surface layer of pyrolytic boron nitride in any desired thickness preferably only about 0.01-0.03 inches.

[0013] Thereafter the machined graphite base plate 10 is again coated with a thin film of a material such as aluminum nitride or another coating of pyrolytic boron nitride to form an overcoat or shell 18 which encapsulates the graphite base 10 and provides a protective coating covering any exposed graphite. Electrical contacts are machined through the coatings to expose the graphite at contact locations for connection to an external power source. Alternatively, electrical contact extensions can be machined into the graphite base 10 at the outset before coating or added prior to the overcoating operation.

[0014] In addition, the pBN coating thickness can be increased to promote thermal uniformity, taking into advantage the high degree of thermal conductivity anisotropy inherent in pBN. It should be understood that additional CVD layers of, eg. pyrolytic graphite, can be added to promote thermal uniformity. 

What is claimed is:
 1. A graphite heater comprising a graphite body configured in a spiral or serpentine geometrical shape defining at least one zone of an electrical heating circuit having opposed ends so as to provide an electrical path for each heating zone through the graphite body from the opposed ends with said graphite body having at least one heating surface, a coating of pyrolytic boron nitride or aluminum nitride encapsulating said graphite body and a surface layer of pyrolytic boron nitride or aluminum nitride disposed on said graphite body such that said heating surface is continuous and solid.
 2. A method for forming a graphite heater from a body composed of graphite, comprising the steps of: coating the graphite body in a CVD reactor furnace with a layer of pyrolytic boron nitride or aluminum nitride; machining the coated graphite body from the side opposite said continuous surface to form a spiral or serpentine pattern extending through the graphite body up to said continuous surface such that a solid continuous surface layer of pyrolytic boron nitride or aluminum nitride of predetermined thickness is left intact covering the surface of the machined graphite body, with the spiral pattern formed in said graphite body having opposed ends adapted for connection to an external power supply and encapsulating said machined graphite body in a material composition selected from the group consisting of pyrolytic boron nitride or aluminum nitride to form a thin film shell of such material composition surrounding said heater. 